Plants, as sessile organisms, require acquiring a variety of information from the surroundings for development and growth in an optimum environment. Particularly, formation of floral buds represents one of the development program processes that signify the dramatic transition from vegetative growth to reproductive growth at the shoot apical meristem. For successful propagation by means of sexual reproduction, the timing that determines this transition is particularly important. For example, flowering in apples occurs after a very long vegetative growth stage spanning 6 to 12 years from the germination of the seeds. While this is genetically determined and the specific mechanism remains elusive, the long flowering time poses a serious obstacle in, for example, the variety improvement of apple.
It has been elucidated that the timing that initiates the flower and bud formation is controlled by both internal factors such as nutritional conditions, circadian rhythm, and plant growth stage, and environmental external factors such as temperature and photoperiod (Non-Patent Document 1). In recent years, diligent researches using the long-day plant Arabidopsis thaliana (hereinafter, Arabidopsis) as a model plant have proposed that the flower and bud formation is determined by four pathways promoting flower and bud formation, specifically photoperiodic, vernalization, gibberellin (GA), and autonomous pathways (Non-Patent Documents 2 to 4). Because the signals in these pathways complement one another as need arises, losing one of the functions does not completely inhibit the flower and bud formation. The signals for flower and bud formation are integrated by FLOWERING LOCUS T (FT) gene and SUPPRESSOR OF OVEREXPRESSION OF CO1 (SOC1) gene, and promote expression of flower-initiating APETALA1 (AP1) gene and LEAFY (LFY) gene and flowering (Non-Patent Documents 5 to 7). FT gene, a pathway integrator gene, is more strongly expressed in the photoperiodic floral bud promoting pathway, and the transcription control of this gene represents the most important step in regulating flower and bud formation.
FT gene was identified in 1991 through the analysis of flower and bud formation delayed mutants (Non-Patent Document 8). It is known that day-length changes are sensed at the leaves through the interaction of circadian clock and photoreceptor. Expression of CONSTANS (CO) gene is induced in the sieve tissue of leaves (Non-Patent Documents 6 and 9). Expression of CO gene promotes FT gene expression (Non-Patent Documents 10 to 14). It has been elucidated that the FT gene expressed in leaves transfers to the shoot apical meristem in the form of FT protein (Non-Patent Document 15). It has been confirmed that the FD gene specifically expressed at the shoot apices encodes a bZIP transcription factor, and that FD protein is localized at the nucleus (Non-Patent Documents 16 and 17). The control target of FD protein is the AP1 gene which regulates the mitosis in floral buds, and the FT protein binds to the FD protein to promote the AP1 gene transcription activity and flower and bud formation (Non-Patent Documents 16 and 18).
It has also been elucidated that the Arabidopsis TERMINAL FLOWER 1 (TFL1) gene, highly homologous to FT gene, represses flower and bud formation, in contrast to FT gene (Non-Patent Documents 19 and 20). This is because TFL1 gene, highly homologous to FT gene, is antagonistic to FT gene, and binds to the FD protein to inhibit the AP1 gene transcription activity (Non-Patent Document 21). It has been demonstrated through creation of a transformant from a TFL1 gene-deactivated mutant that repressing the TFL1 gene expression promotes flower and bud formation (Non-Patent Document 22). In a recent report, a mutant produced by introducing an antisense strand of apple TFL1 (MdTFL1) gene has been shown to actually promote flower and bud formation in apple, presumably through silencing of TFL1 gene (Non-Patent Document 23). The regulation of the gene expression by RNA silencing is considered to provide an effective means for a genetic approach to explain the regulatory mechanism of flower and bud formation.
Apple latent spherical virus (ALSV) is a virus with a diameter of 25 nm, composed of a segmented, single-stranded RNA genome (RNA1 and RNA2), and three coat proteins (Vp25, Vp20, Vp24). Aside from apples, the virus is known to latently infect five species of Solanaceae plants [Nicotiana tabacum cv. Xanthi nc (hereinafter, nicotiana), Nicotiana glutinosa (hereinafter, glutinosa), Nicotiana occidentalis (hereinafter, occidentalis), Nicotiana benthamiana, Petunia], and Arabidopsis (Non-Patent Document 24). ALSV systemically infects the experimental plant Chenopodium quinoa (hereinafter, quinoa), and causes symptoms of vein clearing and chlorotic mottles (Non-Patent Documents 25 and 26), and symptoms of chlorotic mottles in soybeans in early stages of infection. There have been reports of infectious cDNA clones that include repeated protease cleavage sites between the ALSV-RNA2 product intercellular movement protein (MP) and Vp25, and in which a foreign gene transfer site is added (Non-Patent Documents 27 to 30). Expression of a foreign gene in infected plants using such clones is also reported (Patent Document 2, Non-Patent Documents 27, 31, and 32). ALSV has a highly advantageous characteristic as a virus vector, because the virus is capable of latent infection in nearly all hosts including apples. ALSV thus has great potential in many applications, including introduction and expression of various useful genes, post-genomic analyses using VIGS, and apple breeding using an original host.    [Patent Document 1] JP-A-2008-211993    [Patent Document 2] JP-A-2004-65009 (Expression of Foreign Gene in Apple)    [Non-Patent Document 1] Hastings M H, Follett B K. 2001. 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